The field of the invention is programmable controllers such as those described in U.S. Pat. Nos. 3,810,118; 3,942,158; and 4,165,534 and copending U.S. Pat. No. 4,442,504 entitled "Modular Programmable Controller."
Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with a stored program. In programmable controllers such as those disclosed in the above cited patents, for example, the control program is stored in a memory and includes instructions which are read out in rapid sequence to examine the condition of selected sensing devices on the controlled equipment and instructions which energize or deenergize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
The processor in a programmable controller is designed to rapidly execute programmable controller type instructions which call for the manipulation of single-bit input data and the control of single-bit output data. Such instructions have become quite standardized in the industry in terms of the functions they perform. They may be directly associated with elements of a ladder diagram and are, therefore, easily understood by control engineers who use programmable controllers. Program panels such as those disclosed in U.S. Pat. Nos. 3,798,612 and 3,813,649 and in U.S. Pat. No. 4,070,702 have been developed to assist the user in developing and editing control programs comprised of programmable controller type instructions.
Program loaders typically include an alpha-numeric keyboard and a cathode ray tube (CRT) display. By depressing keys on the keyboard, the user creates a control program which is stored in the programmable controller memory as a series of instructions and which is displayed on the CRT as elements in a ladder diagram. The CRT thus provides a graphic representation of the control functions performed by the stored control program.
Although the ladder diagram format is a very graphic technique for displaying the functions performed by the stored control program, it has its limitations. This is particularly the case with programmable controllers having expanded instruction sets that contain higher level instructions which do not have equivalent ladder diagram elements.